The invention belongs to the telecommunications field. More specifically, the invention pertains to a digital telecommunications system with one or more cordless mobile parts. The mobile parts and the base station exchange communication signals according to the TDMA method.
A telecommunications system of this type is, for example, a radio system operating according to the DECT standard.
The fundamental structure of such a radio system is illustrated in FIG. 2.
The system shown in FIG. 2 comprises a base station B and a plurality of cordless telecommunications terminals TE1 to TEn. In the exemplary embodiment, the cordless telecommunications terminals TE1 to TEn are mobile telephones which are able to communicate via radio with the base station B.
Instead of the cordless telecommunications terminals TE1 to TEn, or in addition, it is also possible to use cordless connection boxes for the connection of corded telecommunications terminals. The cordless telecommunications terminals, the cordless connection boxes and comparable devices are the mobile parts mentioned above in the introduction.
The data transmission between the base station B and the mobile parts is carried out in units of so-called frames, more precisely TDMA frames. The acronym TDMA stands for xe2x80x9cTime Division Multiple Accessxe2x80x9d and states that the frames are constructed such that the base station can communicate with all the mobile parts registered with it in order in successive time slots (or slots) of a respective frame. The communication is effected in each case under the utilization of the full bandwidth of the transmission channel. The TDMA method is sufficiently known and requires no further explanation. The fundamental structure of a (TDMA) frame that is suitable for implementing the TDMA method is explained below with reference to FIG. 3.
As is evident from FIG. 3, such a frame, more precisely the DECT full-slot frame considered in this case, is composed of 24 time slots or slots (full slots) of identical length. The first 12 of the 24 slots are transmitted from the base station to the mobile parts, and the subsequent 12 slots are transmitted from the mobile parts to the base station. More precisely, the zeroth slot of each frame is transmitted from the base station to a zeroth mobile part, the first slot is transmitted from the base station to a first mobile part, the second slot is transmitted from the base station to a second mobile part, . . . , the eleventh slot is transmitted from the base station to an eleventh mobile part, and, conversely, the twelfth slot is transmitted from the zeroth mobile part to the base station, the thirteenth slot is transmitted from the first mobile part to the base station, the fourteenth slot is transmitted from the second mobile part to the base station, . . . , and the twenty-third slot is transmitted from the eleventh mobile part to the base station.
A frame or the 24 slots of a frame are transmitted within 10 ms. Each slot comprises 480 bits and is transmitted in about 417 xcexcs (in 416.66 xcexcs). As is indicated in FIG. 3, the 480 bits are distributed between a sync field having a width of 32 bits, a D field having a width of 388 bits, a Z field having a width of 4 bits, and a guard space field having a width of 56 bits.
320 bits are allocated within the D field for the transmission of the useful data actually of interest (for example voice data). Thus, within 10 ms, the base station can transmit useful data comprising 320 bits to each of the mobile parts and receive the same volume of useful data from each of the mobile parts. The transmission rate for useful data between the base station and each of the mobile parts is thus 32 kbit/s in each direction.
The DECT standard thus makes it possible to realize high-quality systems that can be used quite flexibly.
However, systems of that type are operated as efficiently as set forth above only in very rare cases. This is because in practice the base station and the mobile parts usually operate according to the so-called slow hopping principle, as a result of which the number of radio parts which can be operated from a base station is halved. The reason for this is that only every other slot can be used for the data transmission in the case of slow hopping. The remaining slots are used as so-called blind slots and are allocated for synchronization or readjustment of the transmitting and/or receiving frequency oscillator.
Use of the slow hopping principle mentioned proves to be advantageous because the base station and the radio parts can be constructed in a relatively simple manner on account of the long synchronizability or readjustability of the transmitting and/or receiving frequency oscillator. However, it is unfavorable insofar as the capability of expanding the system is considerably restricted as a result.
It is accordingly an object of the invention to provide a digital telecommunication system, which overcomes the above-mentioned disadvantages of the heretofore-known devices and methods of this general type and which permits the connection of a maximum number of mobile parts, while retaining a simple structure and a high transmission quality.
With the foregoing and other objects in view there is provided, in accordance with the invention, a digital telecommunications system, comprising:
a base station and at least one wireless mobile part adapted to communicate with one another according to a TDMA method with TDMA frames;
one of the base station and the at least one mobile part being programmed to communicate in time slots of mutually different lengths alternately succeeding one another within a TDMA frame.
In accordance with an added feature of the invention, the time slots in the TDMA frame comprise relatively shorter time slots and relatively longer time slots, and wherein the relatively shorter time slots are used for synchronizing or readjusting a transmit and/or receive frequency oscillator of the base station and/or the one or more mobile parts.
In accordance with an additional feature of the invention, a duration of the relatively shorter time slots in the TDMA frame is equal to a duration of a blind slot in a telecommunications system operating according to the DECT standard.
In accordance with another feature of the invention, the relatively longer time slots are data transmission time slots used for data transmission between the base station and the mobile parts.
In accordance with a further feature of the invention, the novel telecommunications system, i.e. the base station and the at least one mobile part, are adapted to operate in the 2.4 GHz ISM band.
In accordance with a concomitant feature of the invention, the base station and the at least one mobile part are modified DECT standard telecommunications system components, modified to allow the TDMA frames with the alternatingly shorter and longer time slots.
As noted, of the time slots of different lengths in each TDMA frame, the shorter time slots are used for the synchronization or readjustment of the transmitting and/or receiving frequency oscillator of the base station and/or of the mobile parts, and the longer time slots are used for the data transmission. It is thereby possible to reduce the proportion of time which is available per unit time for the synchronization or readjustment of the transmitting and/or receiving frequency oscillator. This, correspondingly, increases the proportion of time available per unit time for the data transmission.
The more time is available per frame for the data transmission, the more mobile radio parts can be connected to a base station (with conditions that are otherwise unchanged).
On the other hand, however, the reduced proportion of time for the synchronization or readjustment of the oscillator need not in any way have a negative effect, since the time periods available for synchronization or readjustment and also the intervals between these time periods do not thereby automatically become too short or too long for proper synchronization or readjustment. Apart from this, the reduction in the time available for synchronization or readjustment can be carried out in steps of greater or lesser increment, in particular in systems having a very large number of time slots or slots per frame, with the result that a setting that satisfies all the requirements can be found without difficulty here.
Consequently, the novel telecommunications system permits the connection of a maximum number of mobile parts in conjunction with, as before, a simple structure and a high transmission quality.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a digital telecommunications system, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.